Electromagnetic (EM) heating can be used for enhanced recovery of hydrocarbons from underground reservoirs. Similar to traditional steam-based technologies, the application of EM energy to heat hydrocarbon formations can reduce viscosity and mobilize bitumen and heavy oil within the hydrocarbon formation for production. However, the use of EM heating can require less fresh water than traditional steam-based technologies. As well, the heat transfer with EM heating can be more efficient than that of traditional steam-based technologies, leading to lower capital and operational expenses. The lower cost of EM heating provides the potential to unlock oil reservoirs that would otherwise be unviable or uneconomical for production with steam-based technologies such as shallow formations, thin formations, formations with thick shale layers, and mine-face accessible hydrocarbon formations for example. Hydrocarbon formations can include heavy oil formations, oil sands, tar sands, carbonate formations, sale oil formations, and other hydrocarbon bearing formations.
EM heating of hydrocarbon formations can be achieved by using an EM radiator, or antenna, or applicator, positioned inside an underground reservoir to radiate EM energy to the hydrocarbon formation. The antenna is typically operated resonantly. The antenna can receive EM power generated by an EM wave generator, or radio frequency (RF) generator, located above ground. The EM wave generator typically generates power in the radio frequency range of 300 kHz to 300 MHz.
As the hydrocarbon formation is heated, the characteristics of the hydrocarbon formation, and in particular, the impedance, change. In order to maintain efficient power transfer to the hydrocarbon formation, dynamic or static impedance matching networks can be used between the antenna and the RF generator to limit the reflection of EM power from the antenna back to the RF generator. As well, the RF generator can be adjusted to limit the reflection of EM power from the antenna back to the RF generator. Such operational adjustments and impedance matching networks increase operational, equipment, and design costs.
To carry EM power from an RF generator to the antenna, RF transmission lines capable of delivering high EM power over long distances and capable of withstanding harsh environments (e.g., such as high pressure and temperature) usually found within oil wells are required. However, most commercially available low diameter RF transmission lines are currently limited to delivering low or medium EM power over long distances and rated for lower pressure and temperature than that usually found within oil wells. High power transmission lines such as rectangular waveguides are too large for practical deployment at the frequency range of interest. The cost of currently available RF generators is also high when measured on a cost per RF watt generated basis.
Antennas are typically dipole antennas, which require an electrically lossless or at least low loss region around the two dipole arms. Methods to provide such a lossless region, such as providing electrically lossless material, providing electrically lossless coatings, or forming a lossless region within the hydrocarbon formation, can be complex, expensive, or time-consuming. Furthermore, antenna components typically require electrical isolation, which adds complexity to maintaining mechanical integrity.
Underground antennas generally have short penetration range and hence most of their electromagnetic power is dissipated within a short distance from the antenna. That is, antennas generally heat formations in the range of less than a wavelength, or a few wavelengths of the operating frequency of the antenna.